The present invention relates to a stapedius muscle electrode for fastening on a human stapedius muscle and for detecting the action currents generated upon a contraction of the stapedius muscle.
The three auditory ossicles, the hammer, anvil, and stirrup, which cause the coupling of the eardrum to the inner ear, are located in the middle ear of a human. For this purpose, the hammer is connected to the inner side of the eardrum, while the stirrup is connected to the oval window of the inner ear. In this way, the hammer can absorb the vibrations of the eardrum and transmit them via the anvil, which is situated between the hammer and stirrup, and the stirrup to the oval window, whereby sound vibrations are generated in the liquid of the cochlea, which pass through the liquid. These sound vibrations set hair cells projecting into the liquid into movement, and the movement of the hair cells triggers nerve impulses, which are finally relayed to the brain to generate an impression of hearing.
Normally, it is desirable, as much as possible, for all of the sound power incident in the auditory canal to be relayed via the auditory ossicles to the inner ear. However, from a specific sound pressure, the undamped sound relay would result in damage to the inner ear. For this reason, a mechanism is provided in the human ear, using which the chain of the auditory ossicles can be influenced in their vibration behavior and the sound conduction provided thereby may be reduced in the event of strong acoustic events. This natural protective mechanism is based on two small muscles which involuntarily contract reflexively upon perception of a high sound level. The so-called tensor tympani muscle attaches on the hammer and tensions the eardrum, while the so-called stapedius muscle engages on the stirrup via a tendon associated therewith and tilts the base of the stirrup in the oval window. In this way, the chain of the auditory ossicles is stiffened, so that the coupling of the eardrum to the inner ear worsens. A part of the sound power incident on the eardrum is reflected on the eardrum and a further part is dissipated into the surrounding bone. The hearing is thus protected within certain limits from damage by excessively high sound pressures.
This reaction mechanism of the hearing to protect the inner ear from damage by excessively high sound levels, which results in the tensing of the stapedius muscle, is referred to as the stapedius reflex. It normally begins at sound levels of greater than approximately 80 to 100 dB and begins approximately 50 ms after the beginning of the sound exposure. The stapedius reflex is significant in medicine because conclusions about the functional capability of the ear may be drawn from an atypical behavior or its complete absence. Thus, the use of the reflex at relatively too low or relatively too high sound levels may indicate a functional disturbance of the ear, while its absence may indicate a loss of hearing related to the inner ear, for example, on the one hand, and a so-called otosclerosis—this is more significant from a clinical viewpoint—on the other hand, i.e., a sound conduction hearing impairment because of a fixation of the stirrup.
In addition, the observation of the occurrence of the stapedius reflex in connection with cochlear implants, which are used in deaf people whose deafness is caused by the absence or the destruction of the hair cells in the cochlea, is of great practical significance. This form of deafness cannot be overcome by conventional hearing aids, which only cause an amplification of the sound waves received by the ear. Rather, the electrical nerve impulses to be relayed to the brain must be artificially generated, which can be achieved by stimulation electrodes implanted in the cochlea, which form a part of the cochlear implant. The sense of hearing can often be at least partially reproduced by cochlear implants if the auditory nerve is intact in such a way that speech understanding is possible. However, it is important that the signal level output by the stimulation electrodes is set and regulated in a suitable way. Because the sound energy perceived by the patient can be concluded from the occurrence of the stapedius reflex, the measurement of the stapedius reflex may be used for the matching setting of the cochlear implant. If a sensor device for recognizing the stapedius reflex forms an integral part of the cochlear implant, self-adaptation of its energy output is possible.
To measure the stapedius reflex, inter alia, electrodes are used, which are brought into contact with the tissue of the stapedius muscle and in this way may pick up the action currents generated upon a contraction of the stapedius muscle and conduct them to a suitable measuring unit. However, the reliable contacting of the stapedius muscle has proven to be difficult, because the majority of it is situated inside a channel concealed in the bone and only a small part of it and its tendon are visible and readily accessible from the interior of the middle ear.
The publication U.S. Pat. No. 6,208,882 describes an array of differently designed electrodes for fastening on the stapedius muscle tissue.
One of these electrodes is formed by a serrated, flat blade, which is guided through a surgically produced (slot) incision in the stapedius muscle and held there by the serrations, which form barbs. This electrode has the disadvantage that the required incision in the muscle and the presence of barbs are connected to significant traumatization. In addition, because of the barbs it is only removable poorly and/or with complications. Alternatively, an electrode of this type may also be fastened in that a drilled hole is generated through the bones laterally delimiting the bony channel, and the blade is guided through the hole and bent upward in the channel, so that it runs between the inner wall of the channel and the stapedius muscle. For more secure fastening, the blade may additionally be bent over the upper edge of the channel. However, the disadvantage also exists in this case of traumatization because of the bone hole. In addition, there is an impedance increase in the remaining bone, and the blade is only controllable with difficulty during the fastening, so that the danger of direct traumatization of the muscle also exists.
Another of these electrodes comprises a biocompatible metal wire, which is flexible or pre-shaped in the form of a hook and which has a small ball on one end and is connected at the other end to a coiled insulated wire for connection to an analysis circuit. During the implantation, the electrode is inserted into the bony channel in such a way that the ball is pressed against the stapedius muscle and clamped between it and the adjoining bone. The correct positioning of this electrode requires special instruments. In addition, the hook shape is relatively bulky and the fastening is not very secure, so that the electrode may be easily dislocated by muscle movements. In addition, the removal of a piece of bone is sometimes necessary.
Further of these electrodes are implemented so that they may be laid around the section of the stapedius muscle protruding from the bony channel and secured. Thus, an electrode is formed by a silicone sleeve which can be clamped around the muscle, on whose inner side electrical contacts are implemented. Another electrode is formed by a hook-shaped or U-shaped pre-shaped wire, on whose ends small balls or loops are located and which may be bent and fixed around the stapedius muscle. These designs are not only complex to handle (e.g., they require a spreading instrument), and have relatively bulky and complex constructions, but rather their use also involves the danger of pressure necrosis.
Still another of these electrodes is formed by a multi-strand, Teflon-insulated platinum/iridium wire. For fastening in the muscle tissue, a piece of the insulation is removed on the front end of the wire and bent backward. Subsequently, a drilled hole is produced through the bone, through which the wire is inserted by a needle into the muscle belly. Upon removal of the needle, the bent-over wire front end acts as a hook which fixes the wire in the tissue. This is also disadvantageously connected to traumatization.
In general, these known stapedius muscle electrode configurations thus have the disadvantages that they are very traumatic, require drilled holes in bone, can only be implanted using special instruments, and/or have inadequate fixing upon muscle movements, and are difficult or impossible to remove again.